A bi-directional hydraulic power system may comprise a reservoir, a bi-directional pump, at least one hydraulic component that receives and returns hydraulic fluid, and a plurality of fluid lines connecting these components in fluid communication with one another. By driving the bi-directional pump to rotate in a first direction, hydraulic fluid is moved into a first part of the hydraulic component to apply hydraulic pressure thereto, thereby causing the hydraulic component to perform a movement. This forces hydraulic fluid to be expelled from a second part of the hydraulic component to circulate within the hydraulic power system. Similarly, driving the pump in the opposite direction will move hydraulic fluid into the second part of the hydraulic component, causing the component to move in the opposite manner and to expel fluid from its first part into the hydraulic power system.
In most bi-directional hydraulic power units, hydraulic fluid flow returning to the system from the hydraulic component or components is passed directly to one of the pump inlets. If air is entrained in the return flow of hydraulic fluid, the air will be passed back into the pump and will not be vented to the atmosphere from the hydraulic system. Air in the hydraulic system causes several problems, including jerky movement of components, “spongy” components, air lock of pilot operated check valves, as well as noisy pumps and valves.
Previous attempts to obviate this problem have included pre-filling the hoses to be used in the hydraulic system, and bleeding air from the hydraulic system. Pre-filling hoses and bleeding air often results in spillage of hydraulic oil which can be an environmental hazard if not contained properly. In addition, a user must generally bleed the air from the highest points in the hydraulic power system, which points are not always accessible once the system has been installed in the application. Furthermore, both of these methods are manual and time consuming.
Another method that has been used in the prior art is to install air vent cartridge valves in the hydraulic system. However, such air vent cartridge valves must be external to the power unit and require additional connections. Furthermore, such an approach requires two valves, one each side of the hydraulic component, which adds additional cost to the system. Moreover, air vent cartridge valves are meant to work on the high-pressure side of the system, not on the low-pressure return side, and therefore will not allow air to be purged from the return flow of hydraulic fluid. Thus, hydraulic fluid having air entrained therein will still be passed back to the inlet of the pump, and pump and valve noise, as well as possible air lock of pilot operated check valves, remains a problem